In recent years, in hygiene products such as disposable diapers, sanitary napkins and incontinent pads, the water-absorbing resin as a composition material thereof has been widely used, in view of absorbing a body liquid. As such a water-absorbing resin, for example, there have been known a cross-linked product of partially neutralized polyacrylic acid, a hydrolysate of a starch-acrylic acid graft polymer, a saponified product of a vinyl acetate-acrylate ester copolymer, a hydrolysate of an acrylonitrile copolymer or an acrylamide copolymer and a cross-linked product thereof, and a cross-linked product of a cationic monomer and the like. This water-absorbing resin may be used after converted to also a sheet-like, fiber-like or film-like form, and however, it is generally used after converted to a powder-like form (particulate). As such powders (particles), for example, the particulate water-absorbing agent, having a weight average particle diameter thereof of about 200 to 800 μm, is used widely.
The particulate water-absorbing agent is produced via the polymerization step, the drying step, the crushing step, the classification step, the surface cross-linking step, and the like (Patent Documents 1 to 4). In an industrial scale production, a transportation apparatus is used for connecting each step (Patent Document 5). As a transportation system of the water-absorbing resin, mechanical transportation and pneumatic transportation are included (Patent Documents 6 to 9). In mechanical transportation, for example, a conveyor (a belt conveyor, a bucket conveyor, a screw conveyor) is used as the transportation apparatus.
In addition, in recent years, higher property has been required to such a water-absorbing resin, and as a result, to enhance property of the highly water-absorbing resin, for example, to enhance liquid permeation, technologies (Patent Documents 12 to 17) for using various additives (for example, a polyvalent metal salt, a water-insoluble fine particle) to the water-absorbing resin have been proposed.